Bonding Thordon Elastomers using TG-75 Adhesive
Thordon TG-75 adhesive is a high-strength epoxy developed for bonding Thordon materials to metal substrates. The bonding method is an alternative to interference fitting, typically used when: a. the housing is out of round or irregular, b. there are recesses in the housing that would not provide adequate bearing support, or c. the bearing wall is too thin for an interference fit. This article outlines the best practices for mixing, preparing, and applying the TG-75 adhesive.
The strength of the TG-75 bond is highly dependent on surface preparation, adequate mixing, curing temperature and time, application procedures, and the installation of the bearing into housing after application. The metal substrate and Thordon material should be properly prepared for bonding. The metal surface should be machined not smoother than 3.0 micrometers (125 micro-inches) Ra and grit-blasted to increase adhesion. The Thordon bearing should be machined before bonding, and the surface finish of the Thordon bearing should be between 3.0 to 6.0 micrometers (125 to 250 micro-inches) Ra.
The TG-75 adhesive is a two-part system consisting of an accelerator denoted as “TG-75A” and a resin “TG-75B.” These two components should be mixed using a volume ratio of Part A to Part B = 1:2. Improper mixing ratios may result in the adhesive’s failure to cure or cure too quickly. After mixing, the adhesive offers a maximum working time of 60 minutes at 23°C (73°F).
For cylindrical bearings, the bearing should be sized to allow for a 0.25 mm to 0.38 mm (0.010 in to 0.015 in) adhesive thickness. This is an allowance of 0.50 mm to 0.76 mm (0.020 in to 0.030 in) on the outside diameter. TG-75 will fill gaps up to a thickness of 3.0 mm (0.125 in.) but the shear strength of the adhesive decreases at thickness over 0.6 mm (0.025 in).
Provided the adhesive was mixed correctly, the cure time is dependent upon the temperature during the curing period. The recommended curing temperature for TG-75 is 20°C to 40°C (68°F to 104°F). Over 60% of the bond strength is attained in 8 hours at 23°C (73°F), and full cure is reached in 5 days at 23°C (73°F).
In conclusion, proper mixing, preparation, and application procedures are essential to achieve the highest level of bond integrity possible with the TG-75 adhesive. Bonding is a valuable alternative method to interference fitting, and it provides several benefits, including the ability to bond bearings in irregularly shaped or recessed housings and to control tolerances and installed clearances.
A comprehensive overview of Thordon’s Bonding techniques and tips are included in Thordon Elastomeric Bearings Engineering Manual. Click to download the manual.There is also a great video resource on youtube, click on the image below to view it.
Do you have expensive challenges with your bearings and need a creative, cost-effective solution? Allow Millstream Engineering’s team of specialists to lead your group to a successful, economical solution. Contact us today.